Until recently there has been a tendency to confuse the zeolites erionite and offretite. Recent work has shown that they are substantially different. Erionite from Oregon was first recognized as a new mineral in 1898 by Eakie (Amer. J. Sci., 6, 66-1898). He named it after the Greek word for wool because of its fibrous appearance. In 1959 Deffeys (3) (Am. Minerol., 44, 501-1959) reported the zeolite in tuffaceous sedimentary rocks of Cenozoic age in Nevada, South Dakota and Wyoming. Erionite subsequently has been identified from many deposits in the western United States where it occurs chiefly in altered silicic tuffs of upper Cenozoic lacustrine deposits. The largest deposits of erionite are found in the desert areas of southern California, central Nevada, and southeastern Oregon. It also has been found in some parts of Russia. Offretite was first described in 1890 by Professor Gonnard as a new zeolite in amygdaloidal basalt at Mount Simionse near Montbrison. Loire, France. He named it after Professor Offret of Lyons. Except for a probable occurrence in basalt from Palau Island, Carolina Islands, no other occurrence of offretite has been reported,
The relationship between erionite and offretite was investigated first by Hey and Fejer (10), (Minerol. Mag. 33, 66-1962) who concluded that the two zeolites gave identical x-ray powder photographs. Hey and Fejer suggested that only one name was necessary and that the name offretite had priority. The identity of offretite had been misinterpreted earlier by Strunz (1956), who indicated that offretite was identical with phillipsite on the basis of x-ray study of material from Montbrison. Strunz probably examined phillipsite rather than offretite.
Recently, Bennett and Gard (Nature, 214, 1005-1967) proposed a structural basis for distinguishing erionite from offretite. Electron diffraction and x-ray single crystal studies have shown that the unit cell of offretite is hexagonal, with a.sub.O =13.31, c.sub.o =7.59A; that is, with the c.sub.o dimension half that of erionite. There are no systematically absent reflections. Streaks parallel to c* in electron diffraction patterns of some crystals suggested slight disorder due to stacking faults. Patterns from a synthetic specimen, Linde zeolite T, was more strongly streaked, and had diffused maxima which indicated a disordered intergrowth of erionite and offretite.
If is evident that erionite and offretite are two distinct but closely related minerals which can intergrow; both names, therefore, are valid. They can readily be distinguished by single crystal x-ray or electron diffraction analysis, but with less certainty by x-ray powder diffraction techniques. For finely divided samples electron diffraction can distinguish offretite from inter-growths with erionite, but x-ray powder diffraction techniques are inadequate. Using electron diffraction techniques specimens from seven localities were identified as ordered erionite by Bennett and Gard, but the Mount Simionse specimen was found to be the only natural offretite.
The port size is an important factor that influences the catalytic activity of molecular sieves. The port size must be larger than the reactant and produce molecules to allow diffusion of these species to and from the internal catalytically active surfaces. In most hydrocarbon conversion processes of commercial interest, only the large port molecular sieves, with ports of 7 to 10A are useful. This requirement limits the practical catalytic utilization of erionite, with effective pore diameters ranging from 4 to 6A, to molecules in this size range with critical mean diameters.
Offretite is similar to the erionite structure but differs in the stacking sequence and the resulting channel system parallel to the c axis. The secondary building units in offretite also are double six-membered rings and single six-membered rings, but offretite has a stacking sequence of AABAAB. Bennett and Gard proposed a hexagonal unit cell with the space group P6m2. Its unit cell parameters are a.sub.o =13.31A and c.sub.o =7.59A. The c-axis in offretite is half that of erionite. Offretite has wide channels parallel to c formed by rings of twelve tetrahedra. In erionite this channel is blocked by off-set six-membered rings. Summaries of the structural properties of offretite are given in Table 1.
Table 1 ______________________________________ A summary of the structural properties of offretite ______________________________________ Space group - P6m2 - Unit cell dimensions - a.sub.o = 13.29A, c.sub.o = 7.58A (15) Unit cell content - (K.sub.2, Ca).sub.2 Al.sub.6 Si.sub.14 O.sub.40.17H.su b.2 O Stacking sequence - AABAAB Density - Calculated - 2.372 gm/cc Experimental - 2.13 gm/cc (15) Al:Si ratio - 3:7 Packing factor (calculated) - 0.5145 Total pore volume (calculated) - 562.88A.sup.3 Unit cell volume - 1159.4A.sup.3 Calculated accessible pore volume - 337.3A.sup.3 Inaccessible pore volume - 225.58A.sup.3 Free diameter of 12-membered channel-7.6A Free port diameter of six-membered ring -2.5A Free pore diameter of hexagonal prism - 3.3A ______________________________________
R. A. Sheppard and A. J. Gude (Amer. Minerol., 54, 875-1969) reported that the x-ray powder diffractometer patterns for offretite and erionite are similar; however, they differ sufficiently in detail to be distinguishable. FIG. 1 is a diagrammatic representation of the x-ray patterns. Table 2 gives the data for offretite.
Table 2 ______________________________________ X-ray Powder Diffraction Data for Natural Offretite from Montbrison, France (15) dA dA Relative Intensity hkl Calculated Observed I/Io .times. 100 ______________________________________ 100 11.51 11.50 100 001 7.58 110 6.64 6.64 20 101 6.33 200 5.76 5.76 35 111 4.99 201 4.58 4.58 4 210 4.35 4.35 59 300 3.83 3.83 43 002 3.79 211 3.77 3.77 11 102 3.60 3.60 3 301 3.42 3.42 2 220 3.32 3.32 22 112 3.29 310 3.19 3.19 17 202 3.16 221 3.04 311 2.94 2.94 3 400 2.88 2.88 64 212 2.85 2.85 15 302 2.69 401 2.69 2.69 3 320 2.64 2.64 4 003 2.52 410 2.51 2.51 20 222 2.49 321 2.49 103 2.47 312 2.44 411 2.38 113 2.36 203 2.31 500 2.30 2.30 5 402 2.29 330 2.21 2.21 22 501 2.20 213 2.19 420 2.18 2.18 2 322 2.17 331 2.13 2.13 4 303 2.11 2.11 2 412 2.09 421 2.09 2.09 2 510 2.07 2.07 2 223 2.01 511 1.99 1.99 2 313 1.98 502 1.97 1.97 2 600 1.92 332 1.91 403 1.90 004 1.89 430 1.89 1.89 1 422 1.88 104 1.87 601 1.86 520 1.84 1.84 3 431 1.83 1.83 5 ______________________________________
Differences in the observed lines and the intensity of lines are obvious. Differences in the position of lines are not so obvious in the figure but are measurable. The x-ray data are consistent with a space group of P6m2 for offretite and of P6.sub.3 /mmc for erionite. Erionite characteristically has a more complex x-ray powder diffractometer pattern than offretite. Erionite commonly has double or triple lines, whereas, offretite has single or double lines. Observed lines at 9.07A, 7.51A, and 5.34A distinguish erionite from offretite. The two zeolites can be distinguished in mixture by an x-ray powder diffractometer technique using a slow scanning speed of 0.5.degree. 20/min. The cell dimensions for offretite are a.sub.o = 13.29A and c.sub.o = 7.58A. Cell dimensions for analyzed erionite show the following ranges: a.sub.o = 13.21-13.25A, and c.sub.o = 15.04-15.12A. Thus, the a.sub.o dimension of offretite is larger than that of the erionites, and the doubled c.sub.o dimension of offretite is larger than the c.sub.o dimension of erionites.
The new analysis by Sheppard and Gude along with other analyses are given in Table 3.
Table 3 __________________________________________________________________________ Chemical Composition of Erionite and Offretite Offretite Staples Sheppard Ingram Harada Eakle & Gard Hay Eberly Sheppard & Gude France Japan Oregon Oregon Tanzania Oregon California California __________________________________________________________________________ SiO.sub.2 53.0 54.72 57.16 57.40 57.24 60.81 59.16 60.67 Al.sub.2 O.sub.3 18.1 15.24 16.08 15.60 13.93 13.59 13.44 12.90 Fe.sub.2 O.sub. 3 -- 1.04 -- -- 1.85 3.63 1.48 1.35 FeO -- -- -- -- 0.20 -- 0.05 0.09 MgO 2.0 1.17 0.66 1.11 0.15 0.80 0.26 1.09 CaO 4.1 4.32 3.50 2.92 0.00 1.54 0.21 0.65 Na2O -- 1.00 2.47 1.45 6.24 1.90 6.03 4.39 K.sub.2 O 3.6 2.46 3.51 3.40 4.10 7.17 3.29 4.09 H.sub.2 O+ 17.7 19.12 17.30 17.58 8.18 10.57 8.10 7.69 H.sub.2 O - 1.1 7.08 7.43 6.94 TiO.sub.2 -- -- -- -- 0.32 -- 0.15 0.09 P.sub.2 O.sub.5 -- -- -- -- 0.04 -- 0.03 0.02 MnO -- -- -- -- 0.28 -- 0.03 0.03 Total 99.6 99.07 100.68 99.46 99.53 100.01 99.57 100.00 __________________________________________________________________________
The original chemical analysis of offretite published by Gonnard does not accurately characterize the zeolite. The molecular ratio Al.sub.2 O.sub.3 (Ca,Mg,Na.sub.2, K.sub.2).sub.O for zeolites should be unity; however, this ratio for Gonnard's analysis is about 1.5. Thus, the Al.sub.2 O.sub.3 content of Gonnard's analysis is greatly in excess of his reported CaO and K.sub.2 O content. The new chemical analysis of the offretite from the original locality in France, shows that alkaline earths are greatly in excess of alkali and that the molecular ratio SiO.sub.2 /Al.sub.2 O.sub.3 is about 4.97. Gonnard's analysis showed that potassium was the predominant cation and that the molecular ratio SiO.sub.2 /Al.sub.2 O.sub.3 was 4.67. The total water content in the new analysis is very close to that in Gonnard's analysis. The molecular ratio Al.sub.2 O.sub.3 /(Ca,Mg,Na.sub.2,K.sub.2 ) of the new analysis about 1.1, much closer to unity than the previous value. An interesting point is that neither Gonnard's analysis nor the present one shows Na.sub.2 O.
The analyses show that erionite is more silicieous than offretite and that the molecular ratio SiO.sub.2 /Al.sub.2 O.sub.3 and cation contents are variable. Except for the specimen from Maze, Japan the erionites are alkali-rich. The molecular ratio SiO.sub.2 /Al.sub.2 O.sub.3 ranges from 6.03 to 7.98.
The Si/(Al+Fe.sup.+3) ratio for offretite is 2.48, whereas, the ratio for erionite ranges from 2.92 to 3.74. Only the erionite from Maze, Japan has a Si/(Al+Fe.sup.+3) ratio less than three. Looking at the cation contents of erionite and offretite two observations can be made: (1) offretite does not have a sufficiently characteristic cation content to distinguish it from erionite, and (2) the atomatic percentage of potassium ranges from about 25 to 58, a narrow range compared to that of the other cations.
Offretite and erionite both are uniaxial but differ in optic sign; offretite is negative, whereas erionite is positive. Both are elongated parallel to the c crystallographic axis. As the sign of elongation in the uniaxial crystals is the same as the optic sign, offretite has negative elongation and erionite has positive elongation. Thus, the sign of elongation, as easily determined property, seems sufficient to distinguish offretite from erionite. Indices of refraction for offretite are:
.OMEGA. = 1.489 and .epsilon. = 1.486;
birefrengence is 0.003.
Indices of refraction for erionite are:
.omega. = 1.458-1.477 and .epsilon. = 1.461-1.480; bifringence is 0.003 - 0.005 Sheppard and Gude (15) reported indices as low as .omega. = 1.455 and .epsilon. = 1.459for an erionite sample from Lake Tecopa, California. Rare crystals from the Montbrison specimen are zoned from offretite (negative elongation) at the interior to erionite (positive elongation) at the exterior. Except for this erionite from zoned crystals, no erionite has indices of refraction higher than 1.48, and most erionites have indices below 1.47.
FIG. 3 is a plot of the minimum index of refraction versus the Si/(Al+Fe.sup.+3) ratio for offretite and erionites from different localities as given by Sheppard and Gude. Although there is scatter for the erionites, the plot clearly shows a decrease in the minimum index of refraction with an increase in the Si/(Al+Fe.sup.+3) ratio. Other factors like cation and water contents are also known to affect the index of refraction of zeolites and it probably accounts for the scatter of erionites.
The difference between erionite and offretite and their intergrowths and the resulting x-ray powder and single crystal electron data as summarized follow below:
Single crystal electron diffraction patterns have the following characteristics:
1. Fully ordered erionite crystals give sharp spots with 1-odd, weaker than those with k-even. For fully ordered offretite these spots are completely absent. Disordered intergrowths have streaks parallel to c* through the spots with 1-even; diffuse maxima on these streaks, centered on the lodd positions indicate the extent of erionite type stacking. PA1 2. All natural erionite specimens examined were fully ordered; i.e., odd-1 spots in electron diffraction patterns are never streaked parallel to c*. PA1 3. In the natural offretite from Mt. Simionse, France some particles show disorder with streaks parallel to c* on electron diffraction patterns, and some have the diffuse maxima corresponding to a fair degree of erionite type stacking. PA1 1. Photos taken with Philips non-focusing cameras have only three unique reflections with 1-odd: 10.1, 9.16A; 20.1,5.37A; 21.1,4.17A. PA1 2. guinier-type focusing cameras give better resolution,and at least seven lines with 1-odd in erionite were detected: 10.1,9.16A; 20.1,5.37A; 21.1,4.16A; 21.3,3.29A; 31.1,3.12A; 40.1,2.83A; 32.5,2.33A.
X-ray powder photos have the following characteristics:
Once it had been determined that there were substantial differences between erionite and offretite and that offretite had a 12-ringed structure, attempts were made to synthesize it. One successful attempt was reported by Whyte et al in the Journal of Catalysis 20, 88-96 (1971). Another attempt was reported by Aiello and Barrer in the Journal of the Chemical Society (A), 1970 at page 1470. In both cases, the synthesis of offretite took place by using TMA (tetramethyl-ammonium). Offretite made in this way has a number of deficiencies. First of all, the TMA occupies space in the chamber and, thus, inhibits catalytic activity. Secondly, the TMA may take part in the chemical reaction; while it is possible to remove the TMA from the product, this represents another expensive step in the process. Thirdly, the TMA is expensive. These and other difficulties experienced with the prior art processes have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a synthetic offretite free of stacking faults.
Another object of this invention is the provision of a method of producing a synthetic zeolite of the offretite-type inexpensively.
A further object of the present invention is the provision of a synthetic offretite-like zeolite which is free of undesirable organic cations.
It is another object of the instant invention to provide a synthetic offretite having no TMA occupying space in the chamber.
With the foregoing and other objects in view, which will appear as the description proceeds, the invention resides in the combination and arrangement of steps and the details of the composition hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.